The present embodiments relate to wireless communication systems and, more particularly, to the precoding of Physical Downlink Shared Channel (PDSCH) data and dedicated reference signals with codebook-based feedback for multi-input multi-output (MIMO) transmissions.
With Orthogonal Frequency Division Multiplexing (OFDM), multiple symbols are transmitted on multiple carriers that are spaced apart to provide orthogonality. An OFDM modulator typically takes data symbols into a serial-to-parallel converter, and the output of the serial-to-parallel converter is considered as frequency domain data symbols. The frequency domain tones at either edge of the band may be set to zero and are called guard tones. These guard tones allow the OFDM signal to fit into an appropriate spectral mask. Some of the frequency domain tones are set to values which will be known at the receiver. Among these are Cell-specific Channel State Information Reference Signals (CSI-RS) and Dedicated or Demodulating Reference Signals (DMRS). These reference signals are useful for channel estimation at the receiver. In a multi-input multi-output (MIMO) communication systems with multiple transmit/receive antennas, the data transmission is performed via precoding. Here, precoding refers to a linear (matrix) transformation of a L-stream data into P-stream where L denotes the number of layers (also termed the transmission rank) and P denotes the number of transmit antennas. With the use of dedicated (user-specific) DMRS, a transmitter (base station, also termed eNodeB can perform any precoding operation which is transparent to a user equipment (UE) which acts as a receiver. At the same time, it is beneficial for the base station to obtain a recommendation on the choice of precoding matrix from the user equipment. This is particularly the case for frequency-division duplexing (FDD) where the uplink and downlink channels occupy different parts of the frequency bands, i.e. the uplink and downlink are not reciprocal. Hence, a codebook-based feedback from the UE to the eNodeB is preferred. To enable a codebook-based feedback, a precoding codebook needs to be designed.
To extend cell coverage and service over a wide area, employing remote radio heads (RRHs) is beneficial. Multiple units of RRH are distributed over a wide area and act as multiple distributed antennas for the eNodeB. For downlink transmissions, each RRH unit is associated with a unit of transmit radio device—which constitutes to at least one antenna element along with the associated radio and analog front-end devices. Each unit of RRH is positioned relatively far from the eNodeB and typically connected via a low-latency line such as fiber optic link. Some exemplary configurations are depicted in FIG. 1 where six RRHs are utilized. Depending on whether each RRH is equipped with a single or dual antenna elements, up to 12 antenna elements can be supported.
While the LTE cellular standardization along with its further evolution LTE-Advanced (also known as the E-UTRA and further enhanced E-UTRA, respectively) offer a solid support of codebook-based precoding, the current (Rel.10/11) specification only supports precoding for 2, 4, and 8 antenna elements. From FIG. 2, it is expected that the number of transmit antenna elements changes depending on the number of RRHs. While a downlink transmission with more than 8 antennas may not be necessary, six-antenna transmission is easily envisioned and justified if reasonable flexibility is desired.
While the preceding approaches provide improvements in wireless communications, the present inventors recognize that still further improvements in downlink (DL) spectral efficiency are possible when RRH-based configuration is employed. In particular, a 6-antenna precoding codebook design is invented to improve transmission flexibility. Accordingly, the preferred embodiments described below are directed toward these problems as well as improving upon the prior art. While the preceding approaches provide steady improvements in wireless communications, the present inventors recognize that still further improvements in downlink (DL) spectral efficiency are possible. Accordingly, the preferred embodiments described below are directed toward these problems as well as improving upon the prior art.